As in DOS, DOS Emulation includes device drivers for the three "standard" devices, CON, AUX, and PRN. While DOS packages these device drivers in IBMBIO.COM (IO.SYS), DOS Emulation links these into the DOS Emulation kernel (DOSKRNL) to avoid the need for another file.
Unlike DOS, the AUX and PRN drivers do not include support for COM1, COM2, LPT1, LPT2, and LPT3. These latter devices are supported directly by the OS/2 asynchronous (COM.SYS) and printer (PRINT0n.SYS) device drivers.
This approach allows the CON, AUX, and PRN drivers to behave in a highly compatible manner. These drivers issue ROM BIOS calls (INT 10, 14, and 17, respectively) in order to perform their required tasks. At the same time, using the OS/2 device drivers directly for the numbered I/O devices provides higher performance than through the ROM BIOS interfaces, and allows a numbered I/O device to be easily redirected to a remote device on a network, using the underlying OS/2 mechanisms. Hence there is no INT 17 issued when printing on the numbered I/O devices (for example, LPT1, LPT2, etc.) as long as there is only the virtual printer device driver VLPT.SYS installed as the device driver for LPTn devices. If INT 17 is required, for example by a DOS TSR (Terminate and Stay Resident) that intercepts INT 17, the LPTDD.SYS device driver must be installed as well. You can find LPTDD.SYS in the subdirectory \os2\mdos.
Note: Installing LPTDD.SYS will cause printing from a VDM to slow down.
Remember that only PRN redirection is supported. This is achieved by the virtual printer device driver VLPT.SYS, which routes INT 17 and direct hardware printing to LPT1, LPT2, or LPT3 using the OS/2 file system. This is explained in more detail in Device Drivers.